1. Field of the Invention
This invention relates to a high-power semiconductor device sealed within resin, and more particularly to a high-power semiconductor device which can generate an overcurrent detecting signal, requiring no external elements.
2. Description of the Related Art
A conventional high power semiconductor device will be described with reference to FIGS. 1A, 1B, and 1C. The rated current of the device is several amperes and more.
As shown in FIG. 1A, a conductive layer, for example, copper plate 2 is adhered to the surface of insulating substrate 1. Semiconductor element 3 is soldered on copper plate 2. Conductive layers 4 and 5 made of copper plates are adhered to the surface of insulating layer 1 on the both sides of copper plate 2. Electrodes 6 and 7 are formed on semiconductor element 3 and electrically connected to conductive layers 4 and 5, respectively, by fine wires 8 and 9 made of metal, by means of a known wire bonding method or a ultrasonic bonding method. Terminals C, E, and G are adhered to the conductive layers, i.e., copper plates 2, 4, and 5.
In general, the above-mentioned element is sealed within resin, in the final manufacturing step, to protect the element from atmosphere and moisture. FIG. 1B is a perspective view of the final product, in which terminals C, E, and G protrude from the rectangular sealing resin layer. The emitter of semiconductor element 3 is connected to terminal E, the gate thereof to terminal G, and the collector thereof to terminal C, for example, as shown in FIG. 1C illustrating an insulated gate bipolar transistor (IGBT). In this way, high-power semiconductor device 11 is formed.
As shown in FIGS. 2A and 2B, to prevent failure of the device, which may be caused by an overcurrent flowing through the semiconductor element, resistor 12 (see FIG. 2A) or current transformer 13 (see FIG. 2B) is electrically connected to high power semiconductor device 11 enclosed by broken lines, and detects a current. In FIGS. 2A and 2B, reference symbols L and V denote a load and a power source, respectively.
The result of the detection of a current flowing through resistor 12 o current transformer 13 is fed back to a drive circuit (not shown) connected to high power semiconductor device 11, to shut off or reduce the current if an overcurrent is detected.
When resistor 12 detects a current as is shown in FIG. 2A, power of the product of the square of the current and the resistance is lost. To suppress such a loss of power, a resistor having considerably low resistance is used in general. However, a low resistance resistor is not easily available and expensive. In addition, the resistor must be large, so as to withstand to Joule heat generated in the resistor.
On the other hand, since current detection by means of current transformer 13 as is shown in FIG. 2B causes no power loss, the method is widely applied to a high power semiconductor device. However, it is very difficult for current transformer 13 to detect a direct current perfectly. Further, the transformer must be large enough to detect some kind of pulse, and may therefore be expensive.
As described above, the conventional method of detecting a current is disadvantageous in that an external element for detecting an overcurrent must be connected thereto, and the element is expensive. Moreover, since an external element must be used, the entire system including the semiconductor device tends to be large and difficult to handle.